1. Field of the Invention
The present invention relates to an electroluminescent (EL) device having an emitting layer made of an emitting substance, which utilizes an electroluminescence phenomenon that the emitting substance emits light by applying an electric current to the emitting layer. More particularly, it is concerned with an organic EL device in which the emitting layer is made of an organic emitting substance.
2. Description of the Related Art
As such an EL device type, there have been known a device having two layers each made of an organic compound, as shown in FIG. 1. In this device, an organic fluorescent film or emitting layer 3 and an organic positive-hole transport layer 4 are arranged between a metal electrode or cathode 1 and a transparent electrode or anode 2, the two layers being layered on each other. There have been also known an EL device of three-layer structure, as shown in FIG. 2. In this three-layer device an organic electron transport layer 5, an organic fluorescent film 3 and an organic positive-hole transport layer 4, are arranged between a metal electrode 1 and a transparent electrode 2. The organic positive-hole transport layer 4 has a function to facilitate the injection of positive-holes from the anode as well as another function to block electrons. The organic electron transport layer 5 has a function to facilitate the injection of electrons from the cathode. In these EL devices, where a glass substrate 6 is arranged outside the transparent electrode 2, excitons are generated from the recombination of the electrons injected from the metal electrode 1 with the positive-holes injected to the organic fluorescent film 3 from the transparent electrode 2. At the stage where the excitons are inactivated through radiation, they emit light radiating toward the outside through the transparent electrode 2 and the glass substrate 6. It is known that the emission efficiency of the EL device is improved by rising the entering efficiency of charge carriers with the combination of electrodes of both the transparent anode, made of a conductive material with a high work function, and the cathode, made of a metal with a low work function.
For example, FIG. 3 shows an structure of the two-layer organic EL device of an XY matrix adressing type shown in FIG. 1 which shows a sectional view taken across the line AA' of FIG. 3. This EL device is manufactured in such a manner that a plurality of transparent anode lines 2 made of e.g. indium tin oxide (hereinafter referred as "ITO"), a positive-hole transport layer of organic compound 4, an emitting layer of another organic compound 3 and a plurality of cathode metal lines 1 crossing the anode lines 2, are laminated in sequence on a glass plate 6. The regions crossed by the anode lines 2 and the cathode lines 1 both sandwiching the positive-hole transport layer 4 and the emitting layer 3, emit light with the application of charge carriers. One of the crossed regions corresponds to one pixel of the EL device.
A large size display device constructed with a plurality of XY matrix panels or EL devices having many pixels is driven by supplying an electric current between the anode lines 2 and the cathode lines 1 projecting from the periphery of the panel display.
On the other hand, the luminance efficiency of such an EL device is insufficient for practical use even though it emits at a high efficiency. It is still expected to develop an EL device capable of emission at a further high luminance efficiency, although conventional EL devices can emit light at a lower voltage application. There are problems that when the line resistance value of the transparent anode increases, the consumption electric power of the device increases, and the luminance unevenness on the display surface is conspicuous in proportion to the enlarged size of the display device panel and to the accurately miniatured pixels.